Update MediaKeySession lifetime description to be Oilpan reality-based.

Source/modules/encryptedmedia/MediaKeySession.h

 /*
  * Copyright (C) 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 26 matching lines @@
 #include "public/platform/WebContentDecryptionModuleSession.h"
 #include "public/platform/WebEncryptedMediaTypes.h"
 
 namespace blink {
 
 class GenericEventQueue;
 class MediaKeys;
 
 // References are held by JS only. However, even if all JS references are
 // dropped, it won't be garbage collected until close event received or
-// MediaKeys goes away (as determined by the validity of a WeakPtr). This allows
+// MediaKeys goes away (as determined by a WeakMember reference). This allows
 // the CDM to continue to fire events for this session, as long as the session
 // is open.
 //
-// WeakPtr<MediaKeys> is used instead of having MediaKeys and MediaKeySession
+// WeakMember<MediaKeys> is used instead of having MediaKeys and MediaKeySession
 // keep references to each other, and then having to inform the other object
-// when it gets destroyed.
+// when it gets destroyed. When the Oilpan garbage collector determines that
+// only WeakMember<> references remain to the MediaKeys object, the MediaKeys
+// object will be finalized and the WeakMember<> references will be cleared
+// out(zeroed) by the garbage collector.
 //
 // Because this object controls the lifetime of the WebContentDecryptionModuleSession,
 // it may outlive any JavaScript references as long as the MediaKeys object is alive.
 // The WebContentDecryptionModuleSession has the same lifetime as this object.
 class MediaKeySession final
     : public RefCountedGarbageCollectedEventTargetWithInlineData<MediaKeySession>
     , public ActiveDOMObject
     , private WebContentDecryptionModuleSession::Client {
     DEFINE_EVENT_TARGET_REFCOUNTING_WILL_BE_REMOVED(RefCountedGarbageCollected<MediaKeySession>);
     DEFINE_WRAPPERTYPEINFO();
@@ skipping 70 matching lines @@
     typedef ScriptPromiseProperty<Member<MediaKeySession>, ToV8UndefinedGenerator, RefPtrWillBeMember<DOMException>> ClosedPromise;
     Member<ClosedPromise> m_closedPromise;
 
     HeapDeque<Member<PendingAction>> m_pendingActions;
     Timer<MediaKeySession> m_actionTimer;
 };
 
 } // namespace blink
 
 #endif // MediaKeySession_h